The foundation of plant research results is the sampling and determination of the plot size and sample size. This research was conducted to determine the optimal size and number of plots for monitoring vegetation cover and assessing biodiversity in a part of the rangelands of the flood spreading project in the Birjand plain in the spring of 2024. In this study, plots measuring 1, 2, 3, 4, 5, and 6 m2 with the same number (30 plots each) were established along 5 transects of 100 m each to determine plot dimensions. The time required to measure the density for each plot size was recorded, and based on Wiegert's method, relative variance and relative cost were calculated to determine the optimal plot size for vegetation cover in the flood spreading area. After determining the appropriate plot size, 10, 20, 30, 40, 50, and 60 plots were established along 6 transects, and the density, vegetation cover, and numerical biodiversity indices were determined within them. In this study, in addition, the required number of plots was also calculated using Cochran's method. All statistical analyses were performed in R and PAST software. The results of Wiegert's method and coefficient of variation (CV) showed that a plot size of 4 m2 was the most accurate in measuring vegetation cover in the study area. Considering the dominant growth form of the region's plants and to avoid wasting time and reduce costs, a total of 35 plots of 4 m2 are sufficient to determine the percentage of vegetation cover in this area. Additionally, to investigate and calculate species diversity indices in the flood-spreading area, 90 plots of 6 m2 are required. The results of graphical methods (refraction curve and diversity ordering curve) and nonparametric permutation tests showed that density and species diversity, and dominance are affected by sampling effort.